This invention relates to polymer recovery. More specifically, this invention relates to a process for removing solvent from a liquid mixture of polymer and solvent.
Various polymers are produced by procedures in which a solution of the desired polymer has to be sparated into the polymer and the solvent. Particularly, for synthetic rubber production, this solvent removal constitutes a major problem. For high quality polymers frequently high purity solvents are used. The quantity of such solvents lies normally in the range of 3-10 parts by weight of solvent or diluent per 1 part by weight polymer. Essentially complete recovery of the solvent constitutes an important economic consideration.
It is known in the art that the solvent or diluent of a polymer solution can be and preferably is removed by evaporating this solvent. It is also known that a devolatilizing extruder can be used for this purpose. The extruder utilizes, at least partly, mechanically introduced energy to evaporate the solvent. The cost of such an energy input becomes quite high and large capital investments are necessary for installing these devolatilizing extruders.
A general problem in connection with devolatilizing extruders resides in the throughput. The capacity of, e.g., rubber plants is frequently limited by the devolatilizing extruders which constitute a bottleneck in the production. An increased throughput through these extruders would increase the overall capacity of the plant without the necessity of adding further equipment and thus essentially without further capital expenditure. Furthermore, a serious technical problem occurs in connection with devolatilizing extruders. For reasons that are not quite understood, a fairly severe surging of the power input to the motor driving the extruder occurs. To increase throughput and solvent removal rate and to reduce the power surging of a devolatilizing extruder are two of the main problems that are solved by the present invention.
It is thus one object of this invention to provide a process for the removal of solvent from a liquid polymer/solvent mixture.
A further object of this invention is to provide an apparatus for the removal of solvent from a liquid polymer/solvent mixture.
Another object of this invention is to increase the throughput and solvent removal rate of a devolatilizing extruder.
A further object of this invention is to reduce the power surging in the motors driving devolatilizing extruders.
In accordance with this invention, I have now found that by cooling the devolatilizing extruder rather than by heating it, a higher solvent removal rate, a higher throughput and a smaller power surging of the motor driving the extruder can be achieved. Surprisingly, by cooling the barrel of the devolatilizing extruder that has been heated heretofore, the advantages mentioned are achieved. The replacement of steam heating of the liquid polymer/solvent mixture by water cooling in the area where the mixture is subjected to mechanical working results in a more efficient operation of devolatilizing extruders, particularly permitting higher processing rates to be maintained while avoiding motor overloads. Thus the onstream time of the equipment is increased and the loss caused by off-specification rubber is reduced.
The invention achieves a very substantial reduction in power surging of the motor driving the extruder. It is believed that the power surging is associated with the viscosity of the polymer. Particularly for polymer solutions having a very strong change of viscosity associated with a small amount of solvent evaporation the invention is applicable. Although not all the details of the mechanisms involved are fully understood, it is believed that the cooling of the extruder barrel achieves a control of the amount of viscosity change; the viscosity is gradually changed. Because the viscosity is increased, it is believed that the viscosity of the solution in a given section of the barrel is kept essentially constant. The average energy input into the extruder because of the higher viscosity is increased, but the power surging is very substantially reduced.
The present invention, therefore, provides a process for removing the solvent from a liquid polymer/solvent mixture which process comprises introducing the liquid polymer/solvent mixture into an extruder, operating the extruder to volatilize at least a portion of the solvent, cooling the liquid mixture in the extruder and removing solvent vapor and at least partially devolatilized polymer from the extruder.
The term "liquid polymer/solvent mixture" is intended to characterize a liquid comprising a polymer and a volatile solvent; this liquid mixture may be an actual solution of the polymer in the solvent. Frequently, however, and particularly at the end of the devolatilization process, only a small quantity of solvent remains in the mixture. The mixture then is a polymer having some solvent "dissolved" therein.
The process of this invention is applicable to single phase liquid mixtures of polymers and solvents. In particular, however, and preferably, the process of this invention is carried out for devolatilizing mixtures of polymers or copolymers of elastomers and hydrocarbon solvents. The polymers and copolymers of butadiene, isoprene, styrene and methylstyrenes and the respective hydrocarbon solvents, preferably cyclohexane and methylcyclopentane, are particularly preferred. The most preferred groups of these polymers are those having a weight average molecular weight of above about 250,000.
The operating conditions of the process are such that the section of the extruder where the highest amount of mechanical work is consumed, generally the metering section, is cooled in accordance with this invention. This preferably is done by circulating a coolant through a jacket surrounding the respective section of the barrel of the extruder. The cooling is normally not commenced before the liquid polymer/solvent mixture has reached working temperature which is a function of the polymer viscosity, the solids concentration in the feed and the amount of solvent removed in the extruder. This working temperature is normally in the range of 250.degree.-400.degree. F. (121.degree.-204.degree. C.). The coolant introduced into the jacket of the extruder is at a temperature below the working temperature of the polymer, preferably at least about 50.degree. F. below this working temperature.
The extruders useful for the practice of this invention are general devolatilizing extruders. These extruders are commercially available. Usually these devolatilizing extruders are provided with a cylindrical barrel that is jacketed on the outside for the circulation of temperature control fluids through these jackets. Coaxially with the internal cylinder of the barrel a screw is rotatably arranged leaving a gap between the screw and the barrel through which the liquid polymer/solvent mixture is moved. Screw depth is normally in the range of about 0.05 to about 0.5 inches (1.27.times.10.sup.-3 to 1.27.times.10.sup.-2 m). This gap refers to the distance between the surface of the barrel and the base of the screw thread. The gap is therefore half the difference of the barrel internal diameter and the stem external diameter.
The area where most of the mechanical work is put into the liquid polymer/solvent mixture usually has a gap between the stem and the barrel of about 0.05 to 0.5 inches (1.27.times.10.sup.-3 to 1.27.times.10.sup.-2 m).
The screw of the devolatilizing extruder has a configuration which during rotation propagates the liquid polymer/solvent mixture through the extruder. The screw may be and frequently is composed of sections with different depths and with different pitches.
A preferred embodiment of this invention provides for a devolatilization process wherein the size of the motor power oscillations or surging controls the degree of cooling of the liquid polymer/solvent mixture. In this process the surging of the motor after the polymer/solvent mixture has reached its working temperature is measured. A surge signal is generated responsive to this measurement. This surge signal is then converted into a cooling control signal that increases the flow rate of the coolant and/or decreases its temperature when the surge signal indicates a motor power surge above a setpoint and vice versa.
In accordance with another embodiment of this invention, there is provided an apparatus for devolatilizing a liquid polymer/solvent mixture. This apparatus comprises a devolatilizing extruder having a barrel and a screw with a configuration coaxially and rotatably arranged therein, a motor for rotating the shaft and the screw configuration within the barrel, at least one jacket surrounding the barrel at a location where during processing mechanical work is put into the liquid polymer/solvent mixture in order to evaporate solvent and a source of cooling fluid connected to said jacket for circulating said cooling fluid through the jacket and cooling the barrel to remove heat from the liquid polymer/solvent mixture being at this location of mechanical working. This coolant source can in its simplest configuration be a pump circulating water through the jacket.
The preferred embodiment of this apparatus is one where the coolant circulation is automatically controlled. In this embodiment the apparatus is provided with sensing means to sense the power surging of the motor driving the screw and generating a surge signal representative of the average power surging. The apparatus is furthermore provided with control means controlling at least one of the flow rate and temperature of the coolant responsive to the surge signal such as to increase (decrease) the heat withdrawn from the fluid in the extruder when the power surging is higher (lower) than a setpoint.
It has been found very advantageous to circulate cooling water through the jacket of the extruder in the polymer working zone after the elastomer has been raised to working temperature. When the molten elastomer has reached this point, considerable solvent remains in the melt, requiring the further addition of heat. This heat can be supplied in two ways, by indirect heat transfer through the extruder barrel by circulating a hot fluid through the jacket and by mechanical working within this extruder zone. The potential for heat transfer through the barrel is limited by surface area and the poor heat transfer characteristics of the polymer. At the same time the effectiveness of mechanical working is hampered by the reduced viscosity of the hot melt which still contains a substantial amount of solvent. Bu circulating cooling water through the barrel jacket at this point, one sacrifices this means of heat input; indeed, heat is removed to achieve a degree of indirect control of the viscosity of the melt to a level which allows the extruder screw to effectively induce heat throughout the melt by mechanical working and thus effect vaporization of the remaining solvent. Since the vaporization process results in heat removal from the melt, the ideal situation would be to balance the mechanical heating with the two cooling processes, evaporation and indirect barrel cooling, which are proceeding simultaneously in this melt working zone. This is the balance that the invention attempts to maintain. It has further been found that the best measure of success in achieving the formentioned heat balance is by observing the power input to the extruder screw(s).